28 INTERNAL ANAVOMY OF INSECTS. 
appropriate situation. . But if such striking changes in 
the instinct of these tribes can be effected without any 
perceptible alteration in the structure of the nervous 
chord, it is contrary to the received rules of philosophi- 
cal induction to refer to this alteration the changes in the 
instincts of other tribes where it is found. Is it not far 
more probable that this alteration has in fact no con- 
nexion with the changes of instinct, but is solely con- 
cerned with those remarkable changes in the organs of 
sense and motion, which occur in the larva and Imago 
states of the orders in which it is observed? In a com- 
mon caterpillar, the form of the body, the legs, the eyes, 
and other organs of the senses, all strikingly differ from 
those of the imago; whereas, with the exception of the 
acquisition of new wings, a perfect locust differs little 
from its larva: so that we may reasonably expect a 
corresponding change, such as we find it, in the structure 
of the nervous chord of the lepidopterous insect, not 
called for in that of the neuropterous species, in which 
accordingly it does not take place. 
This reasoning, in opposition to Dr. Virey’s theory, 
that the changes of instinct depend on the altered struc- 
ture of the nervous system, becomes greatly strengthened 
when we advert to the higher classes of animals, which 
surely in any investigation of the nature of instinct ought 
to be closely kept m view; for the faculty, though often 
less perfect in them than in insects, is still of the same 
kind, and may consequently be expected to follow the 
same general laws. In a young swallow, for example, 
all its instincts are not developed at once any more than 
in an insect. The instinct which leads it to migrate 
does not appear for some months after its birth, and that 
